1. Field of the Invention
The present invention relates to optical fiber with long period fiber gratings, more particularly to an optical fiber with microhole-structured long period fiber gratings.
2. Description of the Related Art
Long period fiber grating (LPFG) is an optical device that exhibits a periodic refractive index (RI) change in the optical fiber or a periodic geometry structure along the fiber length. LPFG typically has fiber gratings period of approximately from one to a few hundreds μm. Unlike short-period gratings such as fiber Bragg gratings (FBG), long-period gratings do not backscatter the resonance wavelength. LPFG can couple light from fundamental guided core mode to forward propagating cladding mode at the resonant wavelengths satisfying the phase matching conditions. The periodic change in RI or geometric structure in the fiber can be realized by various means such as ultraviolet (UV) light illumination, CO2 laser irradiation, electric-arc discharge, mechanical pressure and femtosecond laser pulse irradiation.
Vengsarkar, et al., “Long-period fiber gratings as band-rejection filters”, J. Lightwave Technol., 14, 58-65 (1996), discloses an ultraviolet (UV) light inscription method for inscribing long period gratings. In particular, Hydrogen loaded single mode fiber is exposed to a KrF laser (248 nm) through an amplitude mask made of chrome-plated silica. The transmission spectrum of the grating is monitored by a broadband source and an optical spectrum analyzer during the grating is being written. The mechanism of this kind of fabrication is to utilize the photosensitivity of the germanosilicate (Ge) fiber to induce a periodical refractive index change in the fiber core. A drawback of this method is that it requires an amplitude mask, which cannot change the period once it is made.
Savin et al., “Tunable mechanically induced long-period fiber gratings”, Opt. Lett., 25, 710-712 (2000) discloses a mechanical bending method for inscribing long period gratings. In Savin et al., a fiber is inserted between a V-grooved plate and a flat plate. Through increasing pressure on the grooved plate, the fiber is periodically micro-bent and thus results in a periodical refractive index change in the fiber because of the photo-elastic effect. The fiber used does not need to be photosensitive and the grating period can be changed by altering the period of pressure added. This kind of method has a certain amount of polarization dependence. The long term stability and the complicated device component are the disadvantages for this inscription method.
Humbert et al., “Electric-arc-induced gratings in non-hydrogenated fibres: fabrication and high-temperature characterizations”, J. Optics A: Pure and Applied Optics., 4, 194-198 (2002), discloses an electrical arc discharge method that uses a splicer to produce a momentary high temperature, which is usually higher than that of the fictive temperature of the glass. Through the mechanism of thermal shock effect and the residual stress relaxation during the temporary high temperature, the refractive index of the glass fiber will decrease locally. The grating period can be controlled by a computer controlled motor. Moreover, this method does not require the fiber to be photosensitive.
Rao et al., “Novel fiber-optic sensors based on long-period fiber gratings written by high-frequency CO2 laser pulses”, J. Lightwave Technol., 21, 1320-1327 (2003) discloses a high frequency (˜kHz) CO2 laser scanning method for inscribing long period gratings. Compared with CO2 laser with low frequency (˜Hz), this method possesses advantages of small laser focus spot, high heating efficiency and therefore a higher inscription efficiency. The major principle is residual stress relaxation, which is the same as that of electric arc discharge. Besides, periodically deformation and material densification may also change the refractive index of the fiber. The period of the grating can be controlled by a computer. This method also is not limited to photosensitive fiber.
Kondo et al., “Fabrication of long-period fiber gratings by focused irradiation of infrared femtosecond laser pulses”, Opt. Lett., 24, 646-648 (1999), and Kalachev et al., “Long-Period Fiber Grating Fabrication by High-Intensity Femtosecond Pulses at 211 nm”, J. Lightwave Technol., 23, 2568-2578 (2005) disclose a focused irradiation of femtosecond laser pulse method for inscribing long period gratings. Femtosecond laser pulses with wavelength of 800 nm and 211 nm have been used to fabricate LPFGs. The mechanism of this method is to induce the refractive index change mainly in the fiber core through a process of multi-photon absorption. LPFGs made by this method have an enhanced high temperature stability compared with that of UV irradiation.
The strong light intensity over ultra-short pulse duration together with good spatial resolution also makes the femtosecond laser a powerful tool for high precision ablation of glass materials, as disclosed in Marcinkevicius, et al., “Femtosecond laser-assisted three-dimensional machofabraction in silica”. The attractiveness of femtosecond laser pulse ablation also lies in the fact that the materials can be removed in a fast and clean manners, with negligible heat affected zones, thus avoiding any significant damages to the underlying substrate. Such clean and high precision femtosecond laser pulse ablation is well suited for micromachining applications in optical fibers.